Naturally-occurring sulfur constituents are commonly present in crude oils that serve as refinery feedstocks. When the feedstocks are converted at the refinery to the various refined products, many of the sulfur constituents are undesirably retained in the refined products. The presence of sulfur constituents is particularly undesirable in liquid hydrocarbon fuels because sulfur compounds are often emitted into the atmosphere as environmental pollutants upon combustion of the fuel. The presence of sulfur constituents is particularly troublesome in gasolines because nearly all gasoline-fueled automobiles in the United States employ a catalytic converter to treat the combustion off-gases from the engine and reduce the level of nitrogen oxide pollutants emitted in the off-gases. If sulfur constituents are present in the off-gases, the sulfur tends to poison the active noble metal in the emission control catalyst, rendering the catalytic converter less effective. Accordingly, it is desirable to reduce the sulfur content of liquid hydrocarbon fuels, and more generally, to reduce the sulfur content of refinery hydrocarbon streams.
Many processes are known for reducing the sulfur content of hydrocarbon streams. Most are catalytic processes performed with particular catalysts, and often performed under relatively severe conditions of temperature or pressure as exemplified by U.S. Pat. Nos. 3,876,532; 5,454,933; 5,057,473; 5,326,462; 4,188,285; 5,423,975; 5,482,617; and 5,401,391. While such processes favorably reduce the sulfur content of the treated hydrocarbon liquid, the severity of the treatment conditions can chemically alter the resulting hydrocarbon liquid in less desirable ways. Although the practitioner can attempt to mitigate the less desirable effects of the severe treatment conditions by process modifications, the effectiveness of such processes for sulfur reduction is extremely specific to the particular catalyst and/or process conditions. Thus, any significant modification of the catalyst or process conditions tends to negatively impact the effectiveness of sulfur reduction.
Fluid catalytic cracking (FCC) is a preferred process from among several conventional refining processes for producing liquid hydrocarbon fuels from refinery feedstocks. However, it has been reported that the FCC process apportions a large fraction of the sulfur constituents in the refinery feedstock to the resulting liquid hydrocarbon fuels. In particular, it has been found that FCC naphtha, a component of gasoline, contributes the largest fraction of sulfur to the total gasoline pool produced from refineries. Thus, numerous treatment options have been investigated for reducing the sulfur content of liquid hydrocarbon products from FCC units as summarized in Gatte, et al., "Influence of Catalyst on Sulfur Distribution in FCC Gasoline", American Chemical Society 203.sup.rd National Meeting, v. 37, n. 1, pp. 33-40, April, 1992. One treatment option is to hydrotreat the FCC feedstock or product. However, hydrotreating substantially increases the hydrogen demand for the refinery and can downgrade reformulated gasoline which is the desired FCC product. Consequently, Gatte et al. focuses on modifying the FCC process itself to reduce the sulfur content of the FCC product. Modification of the FCC process is likewise not an entirely satisfactory solution to the problem of high sulfur content in gasoline because it is relatively costly to modify the FCC process and the outcomes are uncertain.
It is apparent from the forgoing that a need exists for effectively reducing the sulfur content of a hydrocarbon stream. Accordingly, it is an object of the present invention to provide an effective process for reducing the sulfur content of a hydrocarbon stream, and particularly for reducing the sulfur content of refined liquid hydrocarbon products. More particularly, it is an object of the present invention to provide such a process for reducing the sulfur content of liquid hydrocarbon fuels such as gasolines and distillates. It is another object of the present invention to provide such a process for reducing the sulfur content of a hydrocarbon stream under relatively low severity treatment conditions. It is still another object of the present invention to provide such a process for reducing the sulfur content of a hydrocarbon stream which is relatively cost effective. It is a further object of the present invention to provide such a process for reducing the sulfur content of a hydrocarbon stream without substantially modifying existing refinery processes. These objects and others are achieved in accordance with the invention described hereafter.